Electron optical system



p 1947. b. GABOR I ELECTRON OPTICAL SYSTE M Filed Febf 12, 1944 Inventor; Dennis Gabor,

His Attorney.

Patented Sept. 2, 1947 Dennis Gabor, Rugby, England, assignor to Genoral- Electric Company, a corporation of New York ATENT QFF 15C Application February'IZ, 1944} Serial No. 5221 38 In Great Britain March 3', 1943 9 Claims.

This. invention relates to electron optical systems and, more particularly, to systems of this type employed in electron microscopes; It has for its object to provide an improved electron lens arrangement for such a-system'.

In the field of electronmicroscopes, it is known that spherical aberration of the electron lenses used constitutes a limiting factor in the resolu tion power ofsuch microscopes. Efiorts to correct spherical aberration of such lenses have proved fruitless, primarily since there exists no electron-optical equivalent of the dispersing lens or'of' the correcting plates used in light optics for the elimination of this defect. Due to the inability' to correct for spherical aberration, prior to this it has been impossible to make the resolution limit of electron microscopes less than about 30: Angstroms. This limitation is partly due to the impossibility of correcting for spherical aberration which makes it necessary to use extremely small apertures,- and partly also to the practical impossibility of constructing objective lenses having focal lengths less than about 3 millime ter's. When electron lenses-having local lengths of this order are employed, the fluctuations of the energizing voltage, even if these fluctuations are made as small as 002%, by defocusing of the lens produce errors of nearly the sam size as those produced by spherical aberration and diffraction; Further progress in decreasing the resolution limit of electron microscopes, therefore, depends upon the correction of spherical aberration and substantial reduction in the focal lengths of the objectivelenses Accordingly, it is an object of the present invention to provide an electron lens having an extremely short focal length;

Another object of the invention is to provide an' improved electron lens system in which the spherical: aberration of the lenses is substantially reduced.

Still another object" ofthe invention is to providea space chargelens for an electron optical system.

A- still further object of the invention isto provide an improved electron lens for an electron optical system in'which the electrons of an electron stream'passing throu'ghthe lens are focused to: a path substantially parallel with the axis of thestream. 7

Oneof the features of the invention consists inthe improvement of a condensing lens which may be of either the magnetic or electrostatic type by combining therewith a dispersing lens which;- in conjunction with-an auxiliary cathode,

produces a concentrated cloud ofslow electrons along a portion of: the optical axis-and trapping the cloud of electrons within a closed'space by thecombined effect ofelectrostatic and magnetic fields. Y Electrode means are provided to remove electrons from the cloud at the rate they are supplied. by the auxiliary cathode to maintaina desired: density of the concentrated electron cloud.

The featuresof the invention which are believed to'benovel are: set fo'rth with particulari'ty in the appended claims, The invention itself, however, both as to its organization. and method of operation; together withfurther objects and advantages thereof, may best beunderstood by reference tothe following description taken in connection with the accompanying drawing, in which-Fig. l is apart sectionth-rough an electron lens system accordingtoth invention; Figs. 2 and 3- are diagrams illustrating the manner in which a short resulting focal length is producedby the lens. system; and Fig 4 illustrates. the distributionof the/space charge density and potentialin the region ofthe concentrated electron cloud. I

In the part section of the objective lens-of the invention illustrated in- Fig. l, there iss'chematically represented only the upper half of the electron lens system, the lower half being identical thereto, The objective lens asdepicted'compri'ses a coil: or winding I enclosed in a magnetic metallic casing Z to constitute a magnetic lens of well-known type. A stream of high-speed image-bearing electrons represented by the arrow 3 passes-along the axis l of the coil I. A second lens; arranged axially behindthe magnetic lens, comprises a coilor winding 5' enclosed in a metallic magnetic casing 6 and. concentrically surrounding a tubular electrode T. An auxiliary cathode 8,. inthe shape of an annular filament, is disposed near the entrance to. the second lens system and is supplied: w-itnheating current by meansof leads 9 energized from a suitable source of potential. Electrode 1: is maintained at a positive potential with respect to cathode 8 by means of lead 'lllconnect'ed to a suitable source of potential.

Theelectronv lens arrangement. illustrated in Fig-.. 1 may be empleyedin an. electron microscope, the remaining portions of whichmay be of any well-knownform and are not illustrated. In the usual non-self lum'inou type or electron microscopethe image i'stormed by electrons, produced by a separate cathode, which strikes the object under examination after having been accelerated to a rather high velocity. Thereafter, the electron beam 3 is subjected to the eiTects of the lens system and ultimately is directed to either a fluorescent screen or a photographic film,

In a condensing lens of the magnetic type, such as the coil I in Fig. 1, the magnetic field exerts a force at right angles to the path of the electrons of the beam 3 in the field, producing a sort of spiral path for the electrons. This motion can be separated into movement in a meridian plane and rotation around the axis. A magnetic field of this type is always a condensing lens and the trajectory of the electrons is always bent toward the axis.

The action of the second lens in Fig. 1 comprising the coil and the tubular electrode 1 is essentially different from that of the first lens. In this second lens, the magnetic coil 5 produces a long, but weaker, field along the axis of the electron beam 3 than the coil I. The tWo magnetic fields have the same sign but there is a leakage region between the two in which the field strength goes to zero. This field line of zero strength has approximately hyperbolic shape and i indicated in Fig. 1 by the line II. In some instances it may be necessary, in order to produce this zero region, to use a special coil between the two windings I and'5. The auxiliary cathode 8 is located on this line of zero field strength and serves as the electron source for a cloud of lowspeed electrons present within the tubular electrode I, the latter being maintained at a positive potential with respect to cathode 8.

It is known that an electron emitted from a cathode and moving in an axially symmetrical magnetic field acquires an angular momentum relative to the axis. The value of this angular momentum is proportional to the number of lines of magnetic force which intersect the surface'of revolution generated by rotating the trajectory of the electron around the axis. Hence, an electron of the beam 3 which has crossed a certain magnetic flux on its way from the main cathode (not shown) can never reach the axis 4, as at the axis its angular momentum would be zero. In order, therefore, to let electrons reach the axis at all, it is necessary to arrange at least part of the auxiliary cathode 8 in such a position that the flux between this cathode and the axis is zero. This is achieved in the structure of Fig. 1 by placing the cathode 8 at or near the surface of revolution II at which the magnetic intensity is zero.

If the magnetic field set up by the coil 5 is of sufiicient strength, the electrons of the cloud within the second lens system are never able to reach the sheath electrode I which has a potential positive with respect to cathode 8. As an electron moves outwards, its angular momentum increases, that is, its kinetic energy is converted partly into rotational energy. If the magnetic field produced by the coil 5 is of sufficient strength relative to the voltage difference between cathode 8 and sheath 1, at a certain radius, indicated by the dotted line I2, the whole movement of the electrons from cathode 8 becomes rotational, that'is, at right angles to the plane of the drawing in Fig, 1. Under such conditions, the electrons cannot approach the sheath, electrode 1 any further. Means are also provided to maintain the cloud of electrons within the tubular sheath I by providing electrostatic forces at the ends of this sheath. To this purpose, the casings 2 and 6 are connected by means of conductors I3 and M with otentials sufficiently below the potential of cathode 8. Alternatively, special electrodes may be provided at the ends of sheath I to effect the same result.

Since the electrons of the cloud within the tube 7 cannot escape to any of the outer electrodes because of the magnetic and electrostatic forces, they can escape only by returning to the cathode 8. This, however, is a slow process for two reasons. First, the area of the cathode B is very small compared with the area of the boundary I2. Secondly, a returning electron can reach the cathode only if it approaches it at very nearly right angles, else its angular momentum will again carry it away from it. It is therefore seen that the electrons emitted by the cathode 8 are trapped in the region defined by the surface I2 and escape only after they have traversed it a multitude of times. It is therefore possible to trap inside the region I2 two very large equal and opposite electron currents, one flowing to the right and the other to the left, each being many times larger than the emission of the cathode 8. The axial currents cancel one another, while the rotary currents connected to the two streams have the same sign. This electron current rotating around the axis has a sign opposite to that of the current in the coil 5 which produces the magnetic field, that is, the rotating electron current tends to weaken the field near the axis. This has a beneficial effect from the point of view of the invention, as the magnetic field has a condensing lens effect and the new lens begins to act as a diverging lens only if the space charge efiect outweighs the magnetic effect.

If the trapped electrons were allowed to accumulate in the enclosed space until their space charge repelled any further electrons emitted by cathode 8, a static cloud of electrons would be produced. Suitable electrodes, therefore, such as the electrode I6, may be used to remove the electrons at a certain desired rate, and by this to regulate the concentration of the space charge. The electrodes I6 preferably are kept at potentials only a little higher than the regular cathode potential to avoid undue heat development.

The effect of the lens system as described on the fast electrons used for microscopic image formation is illustrated in Fig. 2, which shows schematically the distribution of the magnetic field strength H and of the negative space charge density p along the axis. The portion 20 of this curve shows the magnetic field strength within the lens I and illustrates how this magnetic field is Zero at the surface of revolution II. The region 2! shows the negative space charge density within the tubular electrode I.

In Fig. 3 there is shown the trajectory of a fast electron emitted from the primary cathode (not shown) and used for microscopic image formation, the distance 1' indicating the distance of the electron from the axis 4. The electron is bent toward the axis 4 by the condensing lens constituted by the magnetic coil I and away from this axis by the space charge lens comprising the cloud of electrons within electrode 1 until it becomes approximately parallel to the axis near the end of the space charge lens. The lens system illustrated in Fig. 1 combines to form a short focus lens which is a combination of the long magnetic lens 2 and the long diverging space charge lens. The focal length of this lens is the projection on the axis 4 of the intersection of the initial tangent 22 and the final tangent 23 to the trajectory. It is therefore possible that a this focal length can be made much smaller of zero field strength and by 55 than the longitudinalpextensionzof the-lenses and it can he reduced to almost any small value if the "space charge lens is made sufiiciently long. The focal length f 'is approximately an exponential function of this iength ofthe space-charge lens.

In Fig. 4,1the'upper portionof the figure illustrates the density distribution within the electrode l and shows the manner in which the region l2 containsacloud of electrons surrounding the axis 4. The lower portion of Fig. 4 illustrates the space jcharge distribution ,p and the potential distribution. in this region. The --desired shape for the electronlens is achieved only by suitable injection of electrons. This shape, 'shownin Fig. 4, is obtained-'byshapingand positioning the auxiliary cathode -8 on the "surface suitably regulating the injection of the electrons. The electrons accumulate in the tubular space l2 until they have depressed the potential of the axis 4 to the potential of the cathode 8. Beyond this point, no more electrons can reach the axis. In order to prevent the beam from hollowing out and reaching a static condition, the electrons must be removed at a sufiicient rate by means of the electrodes It.

From the foregoing description of the invention, it may be seen that the electron lens illustrated and described may be constructed to have an extremely short focal length. Because of this extremely short focal length of the lens system, in an electron microscope using such a lens system, larger numerical apertures may be employed, the spherical aberration of such larger apertures being corrected by the space charge lens of the lens system. By thus permitting the use of larger apertures and providing lenses of shorter focal length, the invention permits the construction of electron microscopes having a resolution limit which is less than any heretofore obtained.

Moreover, space charge lenses as described above may be used to achromatise an electron optical system, that is, to make the focal length independent of the electron velocity.

While only a particular embodiment of the invention has been shown, it will of course be understood that the invention is not limited thereto since various modifications may be made, and it is contemplated by the appended claims to cover any such modifications as fall Within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electron lens for focusing an electron stream used for image formation comprising means for converging the electrons of said stream toward the axis thereof, means for establishing an auxiliary space charge along the axis of said stream, and means utilizing said space charge for diverging said electrons to a path substantially parallel with said axis.

2. In an electron microscope, a lens for focusing a stream of image-bearing electrons comprising magnetic means for converging the electrons of said stream, means for establishing an auxiliary space charge along the axis of said stream, and means utilizing said space charge for diverging said electrons to a path substantially parallel With said axis.

3. A space charge lens for focusing an electron stream comprising means for establishing an equipotential surface, magnetic means surrounding said equipotential surface, and means for in- 6 'jecting an :auxiliary stream of Lelectron within snidequip'ote ntia'l:surfaceltrnform: a: space charge -therein for 'controlling said firstistream.

f4. spacechargelens for iocusingianeelectron :stre'am comprising gmeans for. establishing an equi potentialisur'iiace, .magnetic means fOI'TSLlIIOLlHding isaid equipotential :surface, means i to .iproduce auxiliary cloud of :electrons confined within :said equip'otential :surface, :and means to remove =electronslfromsaidicloud.

Inanolectrommicroscopie, alen-szsystem ifor lanelectron beamused for 1 image formation ,comgorisingiaamagnetic lens :havingaa-lengthalong the axis :of Iisai'd beam :greater than the diameter -o.f sai beam, and :means for =e'stablishing a "space :charge ilen's in iiclos'e proximity to .said magnetic lens and having a length along said axissubstanti-allyequal to the length of said magnetic lens.

6. In combination, means forforming a primary beam ofelectronsspace charge lens means for focusing said beam of eleetronsecomprising said tubular electrode surrounding said beam, cathode means to inject an auxiliary stream of electrons within said electrode, said electrode being maintained at a positive potential with respect to said cathode means, magnetic means adjacent said electrode to produce rotation of electrons of said stream within said electrode, said magnetic means being of sufficient strength relative to the potential difference between said electrode and said cathode to prevent said electrons of said stream from reaching said electrode, and means to prevent the escape of electrons of said stream from the ends of said electrode.

'7. In combination, means for forming a primary beam of electrons, space charge lens means for focusing said beam of electrons comprising a tubular electrode surrounding said beam, cathode means to produce an auxiliary cloud of electrons within said electrode, said electrode being maintained at a positive potential with respect to said cathode means, magnetic means adjacent said electrode to produce rotation of electrons of said cloud within said electrode, said magnetic means being of suflicient strength relative to the potential difierence between said electrode and said cathode to prevent said electrons of said cloud from reaching said electrode, means to prevent the escape of electrons of said cloud from the ends of said electrode, and means to remove electrons from said cloud to regulate the concentration of the space charge thereof.

8. The combination, in an electron microscope having a stream of electrons used for image formation, of an electron lens for focusing said stream comprising magnetic means for converging the electrons of said stream toward the axis thereof, a tubular electrode positioned in proximity to said magnetic means along the axis of said stream, means for producing an auxiliary cloud of electrons within said tubular electrode, said electrode being maintained at a potential positive with respect to said electron producing means, means for producing a magnetic field within said electrode to efiect rotation of the electrons of said cloud, said magnetic field being of sufiicient intensity to prevent electrons of said cloud from reaching said electrode, and means for preventing the electrons of said cloud from escaping from said electrode.

9. The combination in an electron microscope having a stream of electrons used for image formation, of an electron lens for focusing said stream comprising magnetic means for converging the electrons of said stream toward the axis said stream, means located at a point of zero magnetic field between said magnetic means and said tubular electrode for producing an auxiliary cloud of electrons within said tubular electrode, said electrode being maintained at a potential positive with respect to said electron producing means, means for producing a magnetic field within said electrode to effect rotation of the electrons of said cloud, said magnetic field bein of sufficient intensity to prevent electrons of said cloud from reaching said electrode, means for preventing the electrons of said cloud from escaping from said electrode, and means for removing electrons from said cloud to regulate the space charge thereof.

- DENNIS GAB-OR.

REFERENCES CITED The following references are of record in the file of this patent:

8 UNITED STATES PATENTS Number Name Date 2,058,914 Rudenberg Oct. 27, 1936 2,128,597 Ruska Aug. 30, 1938 2,119,679 Litton June 7, 1938 2,266,411 Clavier et a1. Dec. 16, 1941 2,288,402 Iams June 30, 1942 1,941,157 Smith Dec. 26, 1933 OTHER REFERENCES 

